1. Field of the Invention
This present disclosure relates to methods and/or procedures for determining in a non-destructive way the doping profile of a doped semiconductor region. Such a region can be a doped semiconductor layer formed on a high-resistivity substrate.
This present disclosure relates to methods and/or procedures for determining in a non-destructive way the physical properties of a semiconductor region.
2. Description of the Related Technology
The ITRS roadmap highlights the precise characterization of ultra-shallow junctions (USJs) as one of the top challenges for sub-32 nm Si-CMOS technologies. While the semiconductor technology size reaches the limits of the usually used physical and electrical analytical techniques (secondary ion mass spectrometry (SIMS), spreading resistance profiling (SRP), four-point probe (FPP)), alternative candidates, (e.g. scanning spreading resistance microscopy (SSRM)) are destructive and quite slow, e.g. as samples have to be prepared, and therefore prevent any in-line measurements.
Photomodulated optical reflectance (PMOR) is a widely used contactless technique where a modulated-power pump laser is directed towards a semiconductor sample to modify the refractive index profile thereof. This refractive index profile can be modified through generation of excess carriers, also known as the Drude effect, in the sample and/or by temperature effects of the sample under study. A probe laser is also directed to the semiconductor sample where it will be reflected depending on the refractive index profile. By coupling the reflected probe laser signal to a lock-in amplifier, only the variations in the reflectivity of the semiconductor sample induced by the modulated pump laser are measured.
An example of such PMOR technique is the Therma-Probe® technique (TP), which is a high-modulation-frequency implementation of the PMOR technique. In international patent application WO2007028605 titled “A method and device to quantify active carrier profiles in ultra-shallow semiconductor structures”, the TP method is described. The measurement tool used consisted of two lasers, the pump laser and the probe laser, which are both normally incident on the surface of this semiconductor structure. The pump laser is operated at a wavelength λpump=790 nm, with pump power Ppump=13.5 mW and a laser radius Rpump=0.5 μm while being modulated at a 1 MHz modulation frequency. The probe laser is operated at a wavelength λprobe=670 nm, while the power Pprobe is kept constant=2.5 mW and the laser Rprobe=0.5 μm.
When incident on the substrate the pump laser generates different excess carrier concentration in the doped semiconductor structure and in the substrate. This difference in excess carrier concentration is due to the difference in recombination lifetime between this semiconductor region and the underlying substrate and to the electric field at the metallurgical junction between this semiconductor region and this substrate. By modulating the power of the pump laser with a modulation frequency ω=1 MHz, a modulated excess carrier profile is created with two steep changes, respectively at the surface of the doped semiconductor region and at this metallurgical junction.
This modulated excess carrier profile will result in a modulated refractive index profile with similar steep variations; the relationship between both profiles, i.e. refractive index and free carriers, can be derived using the known Drude model. Due to the interference between the surface and metallurgical junction reflection components of the incident probe laser, the modulated reflectance of the probe laser is representative of the semiconductor doping profile.
As the phase shift of the modulated reflectance with respect to the pump power signal also proved to be dependent on the semiconductor dopant profile, two independent signals can be obtained from the reflected probe laser signal. This is enough information to reconstruct a box-like active doping profile with two unknown parameters N (active doping concentration) and Xj (metallurgical junction depth). The generalization of this measurement procedure to more complex profiles uses either a varying maximum pump power (power curves) or a varying distance between the two lasers (offset curves) as to generate more independent signals. The temperature profile in the semiconductor region is linked via an empirical relationship with the refractive index profile.
Photomodulated optical reflectance (PMOR) technique such as the TP technique has shown its very promising electrical characterization capabilities on box-like doping profiles. TP has indeed proved to be able to characterize such box-like doping profiles in one single fast and non-destructive measurement. However, as the relationship linking PMOR signals to the measured doping profiles is not straightforward, the further use of TP for electrical characterization needs a critical modeling step. Yet, so far, the attempts to model quantitatively the PMOR signals on box-like doping profiles have encountered severe problems. In particular, these models fail when the total integrated concentration of the measured semiconductor structure increases, i.e. when either the junction depth or the total dose increases.
The PMOR methods, in particular the TP method, however only allow characterizing an active doping profile, i.e. providing only information of the spatial distribution of those dopant atoms that provided a free carrier. In case not all dopant atoms are activated, only the active component of the doping profile of semiconductor structure can be characterized. For high total integrated dose even this active doping profile can not be determined accurately using the PMOR method. Moreover the PMOR method doesn't take into account the influence of the inactive part of the doping profile on the physical characteristics, in particular the complex refractive index and the lifetime of the doped semiconductor structure. As these physical characteristics are influenced by the degree of activation of the semiconductors sample under study, the relationship between the measured reflected probe signal and the active dopant profile will also depend on the degree of activation.